Proximity scoring system



Dec. 11, 1962 R. v. WERNER ETAL 3,058,470

PROXIMITY SCORING SYSTEM 2 Sheets-Sheet l Filed 061'.. 15, 1958 Dec. 11,1962 R. v. WERNER ETAL 3,068,470

PRDXIIIIIITY SCORING sYsTEI/I i a sQUARING DIFFERENTIATINC DELAY .YMULTIvIBRAToR 33 CIRCUIT CIRCUIT 2 sec ILJL I 52\ PULSE DELAY 4.,CDINCIDENCE MULTIvIBRAToR CIRCUIT 5 usec XJ'LJ'L 4s\ 49\ 50N f53 i DELAYSQUARING DIFFERENTIATIN AND MULTIVIBRATDR 39 CIRCUIT CIRCUIT 25 sCIRCUIT AGC SIGNAL DELAY To RELAY FIGB AMPLIFIER} 7 MULT'VBRATOR 40 lsec soN CIN 524 MC CRYSTAL CRYSTAL l osCILLATDR Y DIDDE Q17 59 MC MIXERPEG.

INVENTORS RoberfKV. Werner By William J. Thompson tats 3,068A70 PRXIMITYSCRING SYSTEM Robert V. Werner, La Mesa, and William J. Thompson, SanDiego, Calif., assignors to Cubic Corporation, San Diego, Calif., acorporation of `California Filed Oct. 13, 1958, Ser. No. 766,999 6Claims. (Cl. 343-12) The present invention relates to a proximityscoring system and, more particularly, to an electronic system forpresenting an immediate indication whenever a tired missile attains apredetermined slant range from a designated target.

One of the major elements constituting the air defense complex at thepresent time is the interceptor aircraft carrying air-to-air missilesfor potential use against enemy aircraft. Although aircraft-missilelaunch techniques have been automatized to a considerable extent,superior pilot skill and judgment are still required in the lastanalysis to attain a reasonably high kill probability. In order forinterceptor pilots to attain this needed degree of skill and judgment,considerable practice is required in launching missiles from theinterceptor at simulated targets, i.e., towed drones, balloons, etc. Animportant aspect of this type of practice is to present immediatefeedback information to the pilot of the results of each firing, thatis, whether the minimum distance reached by the missile from the target,termed target missdistance, was suiciently small to register as a kill,based on the type of missile, warhead and assumed target.

Several systems have been employed, and others proposed for acquiringthis miss-distance information, but in all cases, considerable time lagensues between missile tiring and subsequent indication of the resultsto the pilot. Ground-based systems, for example, can simultaneouslytrack the target, the interceptor aircraft and the missile during itsflight from the aircraft toward and past kthe target. However,considerable time is required after ring to compute miss-distance andrelay the results of the computation to the pilot. For optimum training,however, it is desirable to give the pilot a much faster indication ofthe tiring results in order that, when he fires again, he will stillrecall all details of his previous tiring attempt. The problem isanalogous to a rie marksman, for example, shooting a series of bulletsat a target one day and then being told the history of hits and missesthe next day. Learning, under such conditions, would be greatlyprotracted because of the basic inability to use the immediate resultsof each firing for corrective purposes on the next firing.

The present system serves to give an immediate indication to the pilotof his tiring performance. It includes one electronic unit located inthe target and another, extremely simple transponder unit located in themissile. The target-based portion of the present system transmits acarrier signal frequency-modulated by a range signal which is receivedby the transponder in the missile and returned, at `a different carrierfrequency, to the target aircraft.

The range signal will experience a phase shift traveling to and from themissile transponder whose magnitude corresponds to twice the distanceybetween missile and target. The originally produced or reference rangesignal is, within the target-based unit, eifectively delayed in phase bybeing passed through a resolver. Then, the resolver phase-delayed signalis compared with the phaseshifted range signal acquired'by demodulatingthe received carrier signal from the transponder. Now, the resolvershaft position is preset to a value such that the phase shift introducedin the reference signal in passing through it equals a predeterminedmissile-to-target distance. Then, thephase-delayed reference signal andthe distancedelayed or data signal are vcontinuously compared for phasecoincidence, and, upon phase coincidence, an indicator, for example, asmoke bomb in the target, is immediately tired to thereby present anobvious, 'immediate visual indication to the interceptor pilot that thefiring succeeded and a score was made. Of course, if no coincidenceoccurs, denoting that the minimum missileto-target range exceeded thedistance value initially set into the resolver shaft position, then noscoring indication is generated and the pilot, in this case, receivesimmediate knowledge of a miss.

The description of the signal modulating and demodulating schemeemployed by the present system has been deliberately simplified in theover-all system operational description above but, in practice,represents a major improvement over conventional techniques. Inparticular, a range signal of 2 mc. is phase modulated on the targetcarrier signal and this same range signal frequency is retransmitted bythe transponder but modulated on a different frequency carrier signal.Then, in the target circuitry, a signal produced by a local oscillator,differing in frequency by 10 kc. from the original range signal, ismodulated on a carrier signal whose frequency also differs from both ofthe vtransmitted and transponder return carrier signals. This localoscillator signal is mixed both with the original and the returnmodulated carrier signals and the resulting mixture is kapplied to theinput terminal of each of a pair of respective I-F ampliers.

The result of this mixing action will produce an amplitude-modulatedsignal of l0 kc. superimposed on the two I-F signal envelopes whichcarry the identical phase information relative to each other as do theoriginal Z-mc. range signals. The lO-kc. signals are then detected by apair of A-M detectors, and the referenced derived signal is passedthrough the noted resolver and the data signal is coupled directly to aphase coincidence detector circuit. Then, phase coincidence between thesignals will be detected and a signal representing the detection istransmitted to the indicator portion which then yields a visualindication. By employing this particular modulating scheme, narrow I-Fbandwidth requirements may be maintained, based on the lO-kc. signal,since the phase information appears in them rather than the 2-mc.signals originally used. Accordingly, the I-F circuit requirements aregreatly minimized, and the high accuracy distance information inherentin the 2-mc. signal `phase delay is that effectively measured by thesystem but at the reduced frequency. Hence, circuit simplicity iscombined with high accuracy distance measurement capabilities.

It is, accordingly, the principal object of the present system toprovide a proximity scoring system for missile launching interceptors,where an immediate indication is given the interceptor pilot of theresults of each missile tiring at a target.

Another object of the present invention is to provide a slant-rangemeasuring system for producing an immediate, positive indicationwhenever a predetermined slant range between a pair of moving objects isattained.

vStill another object ofthe present invention is to provide anelectronic system for measuring phase delay in a signal transmittedbetween a pair of objects and presenting a positive indication whenevera predetermined phase delay is obtained.

A further object of the present invention is toprovide a proximityscoring system capable of presenting a positive indication when a vtiredmissile passes within a predetermined slant range distance from a targetby effectively measuring the phase delay incurred in a signaltransmitted from the target to the missile and returned.

A still further object of the present invention is to provide aproximity scoring system which continuously measures the phase delayincurred in a signal transmitted and returned to a target-based systemfrom a missile with a predetermined phase delay representing apredetermined missile-to-target distance, and presenting a positiveindication Whenever phase coincidence between the two signals occurs.

Other objects, features and attendant advantages of the presentinvention will become more apparent to those skilled in the art as thefollowing disclosure is set forth, including a detailed description of apreferred embodiment of the invention as illustrated in the accompanyingsheets of drawings, in which:

FIGURE l is a detailed block diagrammatic representation of theelectronic portion of the present system located in the target;

FIGURE 2 is the circuitry in block diagram form of the phase coincidencedetector circuit of FIGURE l; and

FIGURE 3 is a block diagrammatic representation of the electronicportion of the present system located in the missile.

Referring now to the drawings wherein the same circuit elements aregiven the same numerical designation in the several figures, there isillustrated in FIGURE l, the electronic portion of the present systemlocated in the target or its immediate vicinity. It includes atransmitter which includes a pair of crystal oscillators 12 and 14producing output signal frequencies of 265 mc. and 2 mc., respectively,both being applied to the two input terminals of a phase modulator 15.The output signal of phase modulator 15, constituting the output signalof transmitter 10, is applied to an antenna 16 for transmission toanother antenna 17, carried by the missile, and connected to the missiletransponder 18 whose circuitry is later shown in Iblock diagram form inFIGURE 3. The common line formed by the output terminal of transmitter10 and antenna 16 is coupled to the input terminal of a 32A-mc. passbandfilter 19 and also to one input terminal of a mixer 20. The outputterminal of filter 19 is coupled to one input terminal of another mixer21.

A local oscillator, indicated at 22, includes a pair of crystaloscillators 23 and 24 producing 288-mc. and 1.99-rnc. output signalfrequencies, respectively, which are applied to the two input terminalsof another phase modulator 25. The output signal of modulator 25,constituting the output signal of oscillator 22, is applied to the otherinput terminals of each of mixers 20 and 21.

The output signal of mixer 21 is amplified by an I-F amplifier,indicated at 27, having a center frequency of 36 mc., and is thenapplied through an A-M detector 28 to one input terminal of a phasecoincidence detector circuit, indicated at 32, and shown in detail inthe following FIGURE 2. An AGC signal is taken from l-F amplifier 27 ona separate line 30, and also applied to phase coincidence detector 32.

The output signal of mixer 20 is fed serially through an I-F amplifier34, having a center frequency of 23 mc., an A-M detector 35, anamplifier and phase shifter 36, into a resolver 37, shown in blockdiagrammatic form. The rotor output shaft ofv resolver 37 is associatedwith a dial indicator 38, not shown in detail, and is adapted to bemanually rotated for purposes to be later explained. The output signalof resolver 37 is applied over conductor 39 to the other input terminalof phase coincidence detector 32.

The output signal from detector 32 is applied over an output conductor40 to a relay amplifier 41, whose output signal, in turn, is applied tothe energizing coil of a relay, shown generally at 42. The movablecontact arm of relay 42 is connected to the positive terminal B+ of asource of positive potential, not herein specifically illustrated, andits lower switch point, in turn, is connected to an indicator 43.

In considering the detailed operation of the proximity scoring systemaccording to the present invention, modulator 15 within transmitter 10serves to phase-modulate the 2-mc. output signal of oscillator 14 on the265-mc. output signal of oscillator 12. The output phase-modulatedsignal is routed to antenna 16 for transmission to missile transponder18. As will be later shown in more detail in FIGURE 3, and described inconnection therewith, transponder 18 receives this target-transmittedsignal on its associated antenna 17, mixes it with a 59-mc. signalproduced by an internal crystal oscillator and retransmits the sum andditference frequency signals back to antenna 16. The signal receivedfrom the transponder will include the original 2-mc. modulationcomponent whose phase, however, has undergone a delay corresponding tothe total distance traversed during the course of its transmission, thatis, the distance from antenna 16 to antenna 17 and back again, or twicethe distance between the target and missile. Accordingly, distanceinformation pertaining to the missile-target spacing is found in thephase difference between the 2-mc. modulation component beforetransmission and after its return from the transponder.

Local oscillator 22 produces a 288-mc. carrier signal phase-modulated bythe 1.99-mc. signal of oscillator 24. This local oscillator signal ismixed, by mixer 21, with the transponder return sum-frequency signal of324 megacycles, previously passed by filter 19. The resulting signaldifference frequency of k36 mc., produced by the mixing action, isamplified by I-F amplifier 27.

Now, a unique result is achieved by mixing two carrier signals ofdifferent frequencies, the carrier signals, in turn, being modulated bya respective pair of modulating signals whose freqeuncies also diierfrom each other. Without resorting to thek highly involved mathematicalderivation setting forth the results of such a mixing process, it may bestated that the resulting signal will include, besides the usual mixingcomponents, an A-M signal Whose frequency corresponds to the differencein frequency between the two original modulating signals. Hence, sincethe two modulating signals from crystal oscillators 14 and 24 differ by10 kc., the I-F signal passing through I-F amplitier 27 will contain anA-M signal component of l0 kc.

Now, as may also be shown mathematically, the phase information,produced originally by the target-to-missile travel, contained in thel-kc. signal is based not on its own wavelength, but rather on thewavelength of the original, modulating signal of 2 megacycles. Thisparticular aspect of the present invention is of considerable importancesince, in general, the precision or accuracy to which phase differencemeasurements may be taken between a pair of signals is a strict functionof their basic wavelength accordingly to achieve high accuracymeasurements, it is necessary to employ signals to very shortwavelength. However, the employment of very short wavelength signals,that is, signals of relatively high frequency, imposes wide baudpassrequirements on the I-F amplifier and results in low gain, poor noisecharacteristics, high complexity, etc.

Accordingly, the transfer of the phase information contained in the2-mc. modulating signal into the IO-kc. signal is significantk fromcost, complexity and accuracy standpoints, since the 10-kc. signal phaseinformation may be readily detected to high accuracy, I-F amplifier 27may 4be sharply tuned to acquire high gain characteristics with minimumcircuit complexity without in any way affecting the 10-kc. A-M signalpassing through it.

A-M detector 28 detects the amplitude modulation signal appearing on theI-F signal and applies the resulting lO-kc. data signal to the phasecoincidence detector circuit 32. A l0-kc. reference signal, whose phaserelates to the originally produced 2-mc. modulating signal, is formed-by mixing the 288-mc. modulated carrier signal from local oscillator 22with the transmitter 10 modulated carrier signal of 265 mc., in mixer20. The resulting frequency difference signal of 23 mc. is amplified byI-F amplifier 34 and will contain the lO-kc. A-M component in accordancewith operation described for I-F amplifier 27. This A-M component isthen detected by AM detector 35 and the detected signal amplified andtransformed into +45 and 45 phase leading and lagging signals,respectively, by amplier and phase shifter 36. These two signals arethen applied across the stator windings, not shown, of resolver 37, andthe output signal of the resolver will be taken across its rotorwindings, preferably connected as a linear phase shifter. The lattertype of connecttion may be made by connecting a series resistor andcapacitor across the rotor, the resistor and capacitor having equalvalues at the lO-kc. signal frequency. With the resolver outputconnected as described, the lO-kc. signal appearing at theresistor-capacitor junction will be phase shifted relative to theapplied reference signal an amount exactly equal in degrees to theresolver shaft displacement, away from an initial null position.

The purpose of the present system, as stated earlier, is to give apositive indication if and when the missile reaches a predeterminedscalar distance or slant range from the target. Now, the pasediffe-rence between the -kc. data signal out of detector 28 and theIO-kc. reference signal from detector 35 yields a continuous, directindication of the distance between the missile and the target, based, asnoted earlier, on the phase shift incurred in the 2-mc. modulatingsignal during its travel.

The 2-mc. data signal will undergo a complete cycle of phase delay forevery 500` feet, its wavelength, traveled. This means, then, ignoringfor the moment possible ambiguities, that any distance of the missilefrom the target up to 250i feet may be indicated by the present system,since the data or Z-rnc. signal must travel to the. transponder andreturn. The desired distance indication is accomplished by rotating theshaft 38 of resolver 37 until the phase of the reference signal passingthrough the resolver is delayed an amount equal to the same delay thedata will undergo when the target-to-missile separation equals thedesired distance. It will be appreciated that a scale, marked directlyin feet, may be associated with shaft 38.

Accordingly, when the desired distance is reached, the data signal andthe reference signal, as delayed through the resolver, will have anidentical phase and phase coincidence detector circuit 32 will respondto their phase coincidence by applying an output signal to relayamplifier 41. Relay amplifier 4l will then act to operate relay 42 andthereby couple the positive potential appearing on the B+ terminal toindicator 43.

Indicator 43 may be any one of a number of devices capable of giving arelatively instantaneous indication that the preset range distance fromthe target was attained by the missile. For example, it may be a largeash bulb, smoke bomb, flare, etc., or other type of visual indicator. Aswill be appreciated, the B+ potential will, in most circumstances, beapplied to a suitable triggering device capable of setting off the mainindicating chemical or mechanism.

Referring now to FIGURE 2, there is shown phase coincidence detector 32in detailed block diagrammatic form. In particular, conductor 33 comingfrom A-M detector 2S, not again shown, is applied to the input terminalof a squaring circuit 44, whose output signal is passed serially througha differentiating circuit 45 to the input terminal of a .2microseconddelay multivibrator circuit 46, the output signal of which is applied toone input terminal of a pulse coincidence circuit 47. In the same way,the output signal from resolver 37, not again illustrated, appearing onconductor 39, is applied serially through a squaring circuit 4S, adifferentiating circuit 49, through a 25-microsecond delay multivibrator50 to the other input terminal of pulse coincidence circuit 47.

The output signal of coincidence circuit 47 is applied to the inputterminal of a 5-rnicrosecond delay multivibrator 52, whose outputsignal, in turn, is applied to one input terminal of an and circuit 53.The AGC signal of I-F amplifier 27 of FIGURE 1, appearing on conductor30 is applied to the other input terminal of and circuit 53 and theoutput signal of circuit 53 is applied to the input terminal of al-second delay multivibrator 54. The delay multivibrator 54 outputsignal appears on conductor 40 for application, as before illustrated inFIGURE l, to relay amplifier 41.

Consider now the operation of this FIGURE 2 phase coincidence detectorcircuit. The input signals appearing on input conductors 33 and 38 fromdetector 28 and resolver 36, respectively, will be of sine waveconfiguration and have a frequency of 10` kc. Squaring circuits 44 and48, which, for example may be overdriven class C amplifiers, serve totransform the input sine waves into square wave form, whose leading andlagging edges, in turn, are differentiated by differentiating circuits45 and 49. The .Z-microsecond delay multivibrator 46 responds to eachpositive pulse, for example, appearing in the output signal ofdifferentiating circuit 45 to produce positivegoing signals having aduration of .Z-microsecond. In the same Way, delay multivibrator 50responds to each positive pulse in the output signal of differentiatingcircuit 49 to produce a high voltage level of a ZS-microsecond duration.

The reason for employing different multivibrator delay times in the twochannels is to insure pulse coincidence detection, since the relativemissile-to-target velocity will introduce rapidly changing phase shift,i.e., Doppler effect, between the data and reference signals.Accordingly, if pulses of extremely short duration, say of a.Z-microsecond period, appeared on both lines, a possibility would existthat the pulses might slip past each other, between cycles, without theoccurrence of an exact coincidence.. Accordingly, this type of erroneousoperation is eliminated by making the reference pulses sufficientlybroad that coincidence will occur even under the most extreme relativevelocity conditions and will accordingly be detected, in a manner to beshortly shown.

Pulse coincidence circuit 47, operating essentially as an and logiccircuit, as known in the digital computer art, produces an output signalonly when two output signals appear simultaneously from multivibrators46 and 5d. The duration of its output signal, upon input signalcoincidence, will therefore correspond to the duration of the shortest,or .2-microsecond signal.

Since the leading and trailing edges of the square wave signals producedby squaring circuits 44 and 48 correspond to zero cross-over points intheir associated input sine wave signals, the pulses in thedifferentiated signals produced by differentiating circuits 45 and 49will also correspond to the same zero cross-over points. Consequently,the positive-going signals produced by delay multivibrators 46 and 50will correspond to the zero crossover points made by their respectivechannel input signals, and, specifically will correspond to cross-oversmade as its corresponding input sine wave goes from a negative to apositive polarity. This is true since the delay multivibrators, asspecified, are designed to respond only to the pulses of positivepolarity appearing in the pulse stream produced by its associateddifferentiating circuit.

Coincidence, therefore, between the signals produced by this pair ofmultivibrators signifies that coincidence occurs between thenegative-topositive zero cross-overs of their respective input signals,as taken from A-M detector 28 and resolver 36, in FIGURE l. Accordingly,coincidence detection signifies equal phases between the data signalcarrying range information between target and missile and the referencesignal as delayed by an amount corresponding to the preset position ofresolver 36. Hence, at coincidence, the slant range between missile andtarget equals the predetermined range initially placed in the resolvershaft position, and the output pulse resulting from coincidencedetection, of t2- 7 microsecond duration, is applied to delaymultivibrator 52, and its period is increased thereby to microseconds.

The 5-microsec0nd output signal is then applied to and circuit 52 alongwith the AGC signal from I-F am` plifier 27 in FIGURE 1 in order toeliminate possible range ambiguities. In considering ambiguities, itwill be recalled that the 2-mc. signal is capable of measuringunambiguous range out to 250 feet, since 500` feet corresponded to onecomplete wavelength'and its travel includes the distance to and from themissile transponder. However, as will be appreciated, if the missilewere located at, say, 350 feet from the target, the same identical phasedelay will be exhibited as would be the case at a 100- foot distance,since the additional one-cycle phase delay undergone in the longerdistance would not be detectable. The same effect, of course, is presentin continuing the example in distancesof 600 feet, 850 feet, etc., orintegral multiples of the range signal wavelength. Hence, the receivedsignal will always be ambiguous, since there is no way to determine ifthe phase delay being measured represents true range, or is true rangeplus an integral number of wavelengths of delay.

However, Iby sampling the AGC signal'fro'm the I-F amplifier and usingit as a gating signal, ambiguities may be eliminated, since the returnsignal strength will decrease rapidly, speciiically as the square of thedistance between target and missile. Accordingly, the AGC level is setsuch that the received signalv strength from distances over 250 feetwill be insuicient to open and circuit 53. However, during the linalapproach to the target, the transponder signal strength will beincreased and a suiciently strong AGC signal produced to Voperate andVcircuit 53 with the result that whenever pulse coincidence occurs, theresulting output pulse from the coincidence detector will -be passedthrough and circuit 53 and trigger the lsecond multivibrator 54. Thisoutput signal, in turn, will be of sufficient duration to` activaterelay amplifier 41 and, in the manner previously described, triggerindicator 43.

In FIGURE 3 is shown the transponder circuitry in block diagrammaticform.V The transponder antenna 17 is coupled to a crystal diode mixer 61which, in turn, receives the output signal of a 59mc. crystal oscillator60. The vtarget-transmitted modulated 26S-mc. carrier signal is receivedon antenna 17 and is applied Yto the mixer, where it is mixed with the59mc. signal produced by oscillator 60.. The resulting sum anddifference -frequencies resulting from the mixing action appear on themixer output line and are reradiated from antenna 17 back to'theV targetantenna. Only the 324-mc. output signal of the mixer, including therange signal modulated on it,.is indicated on the drawing, since it isthis mixing product signal which is employed by thetarget circuitry forrange detection purposes. v

As will be appreciated by those skilled in the art, the particularcarrier signal frequencies produced by the target-based transmitter, thelocal oscillator and the transponder circuitry are strictly a matter ofengineering expedience, and other carrier signal frequencies withappropriate I-F amplier frequenciesvcould have been employed withoutinvolving invention and still achieve the results stated. In the sameway, the specific range signal frequency of 2 mc. is a matter ofengineering choice as other range signal frequencies could be employedas determined by the accuracy requirements of the system. Also, the -kc.frequency difference between the transmitter modulating signal and thecorresponding local oscillator modulating signal is also a matter ofchoice, and, as will be appreciated, other frequency difference valuescould be employed without the involvement of invention and still obtainthe A-M signal components having the desired range and reference phaseinformation.

As will also be appreciated, indicator 43, shown in FIGURE l, may takemany forms, and a stepping switch arrangement could be employed with anumber of parallel indicators so that a number of serial indicationsmight be made without having to reset the indicating mechanism aftereach score.

It will also be apparent that numerous modifications and changes may beincorporated in the particular arrangement of circuits constitutting thesystem shown and described, and still accomplish the over-all functionset forth without involving invention. It is also apparent that each ofthe circuits shown in block diagrammatic form, may take any one of manywell-known recognized forms as known in the art and shown in numeroushandbooks, technical books, etc., without the employment of invention.

It will be appreciated, of course, that the foregoing disclosure relatesonly to a detailed preferred embodiment of the invention whose spiritand scope of the invention is set forth in the appended claims.

What is claimed is:

l. An electronic system for producing an indication whenever a missilereaches a predetermined slant range from atarget vehicle, said systemcomprising: iirst means for producing a range signal; means yforproducing a first carrier signal phase modulated by said range signal;means for transmitting the phase-modulated first carrier signal; meansassociated with said missile for receiving the transmitted rst carriersignal and retransmitting a second carrier signal phase modulated bysaid range signal; receiving means at said target for receiving thephase-modulated second carrier signal; first modulating means responsiveto the phase-modulated first carrier signal for producing a rstamplitude-modulated signal whose modulation signal phase corresponds tothe phase of the range signal produced by said first means; secondmodulating means responsive to the signal received from said receivingmeans for producing a second amplitudemodulated signal whose modulationsignal phase corresponds to the phase of the range signal in saidreceived signal; means for shifting the phase of the modulation signa-lproduced by said rst modulating means an amount corresponding to saidpredetermined slant range; and means for producing an output indicationwhen the phase of said phase shifted modulation signal equals the phaseof the modulation signal produced by said second modulating meanswhereby said predetermined slant range is indicated.

2. An electronic system for producing an indication whenever a missilereaches a predetermined slant range from a target vehicle, said systemcomprising: means for producing a range signal; means for producing airst carrier signal phase-modulated by said range signal; means fortransmitting the phase-modulated irst carrier signal; means associatedwith said missile for receiving the transmitted first carrier signal andretransmitting a second carrier signal phase-modulated by said rangesignal; receiving means at said target for receiving the phase-modulatedsecond carrier signal; means for producing a third carrier signalphase-modulated by asignal differing in frequency by a predeterminedamount from said range signal; first mixing means for' mixing thephase-modulatedrst carrier signal with said phasemodulated third carriersignal whereby a first amplitude modulated signal is produced whosemodulation frequency corresponds to said predetermined difference andwhose modulation phase corresponds to the phase of said range signal;second mixing means for mixing the phasemodulated second carrier signalwith said phase-modulated third carrier signal whereby a secondamplitudemodulated signal is produced Ywhose modulation frequencycorresponds to said predetermined difference and whose modulation phasecorresponds to the range signal in said received signal; first andsecond detecting means for detecting the amplitude-modulated signalsproduced by'said iirst and second mixing means, respectively; phaseshifting means for shifting the phase of the detected signal producedbyY said rst detectmg means an amount corresponding to saidpredetermined slant range; phase coincidence detecting means responsiveto a phase coincidence between a pair of input signals for producing anoutput indication representing said coincidence; and means for applyingthe phase shifted detected signal produced by said phase shifting meansand the output signal from said second detecting means to said phasecoincidence detector whereby an output indication is produced upon saidpredetermined slant range being reached between said missile from saidtarget vehicle.

3. An electronic system for producing an indication whenever apredetermined slant range is attained between a missile and a targetvehicle, said system comprising:

means for producing a first modulating signal of a first modulatingsignal frequency; means for producing a first carrier signal of a firstcarrier signal frequency; first modula-ting means for modulating saidfirst modulating signal on said first carrier signal; transmittingmeans'associated with said target vehicle for transmitting the m0dulatedfirst carrier signal produced by said first modulating means;transponder means associated with said missile and responsive to thereceipt of said modulated first carrier signal for transmitting a secondcarrier signal 0f a second carrier frequency modulated by said firstmodulating signal; receiving means associated with said target vehiclefor receiving the modulated second carrier signal; means for producing asecond modulating signal of a second modulating s-ignal frequency; meansfor producing a third carrier signal of a third carrier signalfrequency; second modulating means for modulating said second modulatingsignal on said third carrier signal; first mixing means for mixing themodulated third carrier signal with the modulated first carrier signalwhereby a reference amplitude-modulated signal is produced whosemodulating frequency corresponds to the difference between said firstand second modulating signal frequancies; second mixing means for mixingthe modulated second carrier signal from said receiving means with themodulated third carrier whereby a data amplitude-modulated signal isproduced whose modulation frequency corresponds to the differencebetween said first and second modulating signal frequencies and whosephase relationship with said reference modulating signal corresponds tothe slant range between said missile and said target vehicle; first andsecond detecting means for detecting the amplitude-modulated signalsproduced by said first and second mixing means, respectively; phaseshifting means for delaying the phase of the output signal of said rstmixing means an amount corresponding to said predetermined slant rangebetween said target yand said missile; phase comparison means forproducing an output signal whenever phase coincidence exists between apair of applied signals; means for -applying the output signal producedby said phase shifting means and the detected signal produced by saidsecond detecting means to said phase comparison means whereby an outputsignal is produced by said phase comparison means whenever phasecoincidence occurs between the phase shifted and detected signals; andmeans responsive to the signal produced by said phase comparison meansfor producing an indication of said coincidence whereby saidpredetermined range is indicated 4. The electronic system according toclaim 3 wherein said phase comparison means includes first and secondzero signal cross-over detectionmeans associated with said phaseshifting means :and said second detecting means, respectively, forproducing first and second pulse streams corresponding to the zerocross-over points of the corresponding phase shifted and detectedsignals, respectively, and means responsive to the simultaneousappearance of a pair of pulses in said first and second pulse streamsfor producing an output signal.

5. The electronic system according to claim 4 wherein said transpondermeans includes an oscillator producing an output signal whose frequencycorresponds to the difference between said first and second carriersignal frequencies, diode mixing means for mixing the modulated firstcarrier signal with the output signal produced by said oscillator, andmeans for transmitting the sum and difference signals produced by saiddiode mixing means.

6. The electronic system Iaccording to claim 5, including, in addition,means for inhibiting the operation of said phase comparison means unlessthe signal received by said receiving means exceeds a predeterminedsignal strength whereby slant range ambiguities are avoided.

References Cited in the file of this patent UNITED STATES PATENTS2,248,727 Strobel July 8, 1941 2,631,277 Skoller Mar. l0, 1953 2,632,160Rothacker Mar. 17, 1953 2,768,372 Green Oct. 23, 1956 2,866,373 Doyle etal Dec. 30, 1958

